1. Field of the Invention
The present invention relates in general to disk storage systems and more particularly, to a method and apparatus for concurrently erasing and rewriting servo information on a disk in a hard drive assembly.
2. Description of the Related Art
Disk drives are magnetic recording devices used for the storage of information. The information is typically recorded on concentric tracks on either surface of one or more magnetic recording disks. To facilitate the storage and retrieval of data in an orderly manner, disks are typically organized in blocks called sectors. These sectors are located on the disk by a set of unique specifiers called cylinder (or track), head (or side) and sector number. The disks are rotatably mounted to a spin motor and information is accessed by means of read/write heads that are mounted to actuator arms which are rotated by a voice coil motor. The voice coil motor is excited with a current to rotate the actuator and move the heads.
The movement of the actuator is controlled by a servo system, utilizing servo information recorded on one or more of the magnetic recording disks. By reading this servo information, the actual radial positions of the heads can be determined, and after comparison with the desired head radial positions, control signals can be sent to move the actuator accordingly. Servo information is typically stored on a disk in one of two ways. In the first, a dedicated servo system, a set of several tracks on the disk or the entire disk surface, is reserved exclusively for storing information associated with the characteristic of the particular drive. Such information includes servo parameters and read/write channel parameters. A servo head reads this information to provide a continuous signal indicating the position of the servo head with respect to the disk surface. In the second type of servo system, the embedded servo system, sectors of servo information are interspersed with sectors of data on each disk surface. As a read head follows the data track around, it regularly reads a fresh sample of servo information from each servo sector with which to control its position.
Due to power consumption, cost and throughput concerns, a particular type of embedded servo system, known as the "stagger wedge" servo system is used in the disk drive industry. In the stagger wedge servo system, the servo information written on each head or side of a disk are not aligned. Instead, they are staggered. As shown in FIG. 1, the servo information stored on a track of one disk head is phase shifted from the servo information stored on the same track of another disk head.
For example, before writing the servo pattern to the disk surface, the array of read/write heads must be accurately positioned over the disk, by the master arm servo controller. When the master arm servo controller has attained the desired accuracy, head 0 is first activated and directed to write sector 1, head (or side) 0, track 1, of the disk pack in a hard drive assembly. Upon completion of the sector 1, head 0, servo writing process, head 0 is deactivated. R/W head 1 is then activated and directed to write the servo pattern in sector 1, head (or side) 1, track 1 of the disk pack. Upon completion of the servo writing process, R/W head 1 is deactivated. The same process is continued for R/W head 2 and R/W head 3 in sector 1, head (or side) 2, track 1 and sector 1, head (or side) 3, track 1 of the disk pack respectively. Thus, the servo pattern for all four R/W heads of the disk pack are written during a particular sector cycle.
By activating one R/W head at a time, the stagger wedge servo writing scheme may be used to efficiently complete servo writing an entire cylinder with minimum power consumption. However, this servo writing scheme presents a significant problem when used to rewrite servo information on a disk pack in a hard drive assembly. In particular, since only one R/W head (for example R/W head 1) is turned on at a particular time, the remaining heads (for example, R/W heads 2-4) are inactive. As a result, the data fields in sector 1, heads 2-4 of the disk pack will not have any data recorded on them. When a new servo pattern is written over the disk pack, two servo patterns, the old and the new patterns, will co-exist on the disk pack, as shown in FIG. 2, resulting in confusion during an attempt to read the reworked disk.
One approach in solving this problem is to erase the entire disk pack prior to rewriting servo patterns on the disk pack. However, this significantly reduces throughput and thus, the manufacturing productivity of hard disk drives.
Accordingly, there is a need in the technology for a method and apparatus for rewriting servo information on a disk pack in a hard drive assembly without first having to erase the entire disk pack.